Formerly, first responders had been advised to check suspected cardiac arrest victims for a pulse. However, the accurate determination of a pulse has been shown to be poor and the time taken far exceeded the value of the determination. For this reason, lay responders are advised not to administer pulse checks in accordance with European and US Guidelines. If a patient presents with ventricular fibrillation, a pulseless arrhythmia, defibrillation is required immediately. However, if a pulse is present during an episode of High Rate Ventricular Tachycardia (HRVT), then it is possible that a shock might not be the most appropriate therapy.
It is therefore becoming essential that lay persons using public access defibrillators are provided a more reliable means of determining if there is a pulse. Such a means is provided by the measurement of the impedance electrocardiogram—the resistance of the body to the passage of electricity. The impedance Z, as well as its first derivative (dZ/dt), has been shown have direct correlation to arterial blood flow. Herein, the term ICG will mean the first derivative of the patient impedance Z, if necessary after conditioning and filtering.